The present invention relates generally to a steering system and, more particularly to an electrohydraulic steering system for agricultural machines (e.g. tractors, forage harvesters or combine harvesters).
A known steering system is used on forage harvesters for the corn crop. That steering system includes an automatic steering mode (autopilot) to facilitate and optimize working in the field, and a manual steering mode via a steering wheel for road travel. The automatic steering has mechanically deflectable switches as automatic steering signal transmitters for orientation when driving in the rows of corn. An electrical steering signal nominal value is generated each time upon deflection by contact with the corn stalks. This steering signal nominal value is fed to an electronic control and analysis device. At the same time the control and analysis device also receives a wheel position, or steering angle, signal actual value of the steered wheels as determined by a sensor. The control and analysis device compares the wheel position actual value with the automatically generated steering signal nominal value and generates a trigger signal for an electrically operated hydraulic control valve to regulate flow of hydraulic fluid to the steering cylinder which moves the steerable wheels. Thus the steerable wheels are moved or steered by the steering cylinder according to the automatically generated steering signal nominal values in such a way that the machine moves optimally along the corn rows for harvesting use.
The manual steering of the forage harvester is a so-called hydrostatic steering system of rotary slide valve design. Turning the steering wheel rotates the rotary slide valve which is mechanically coupled to the steering shaft, and opens a control port which releases flow of hydraulic fluid to the steering cylinder.
However, such hydrostatic manual steering has some drawbacks. First the design is relatively elaborate because, on account of the relatively long distance between the steering wheel/steering shaft and the axle where the steering cylinder is located, correspondingly long hydraulic pipes have to be installed. Second, on account of the necessarily existing overlap of the rotary slide valve for the control port, hydrostatic steering has a relatively high steering play of up to .+-.5.degree.. Further there is a time delay between the start of steering wheel movement and the steering cylinder movement. This, when added to the amount of the above-mentioned steering play, creates a dynamic behavior which is difficult to influence given the nature of the hydraulic circuitry and the hydraulic components used. Optimization can be obtained only with difficulty by adjustment of the hydraulic components.
The dynamic behavior is such that it is difficult for the operator to drive exactly straight ahead, which at relatively high travelling speeds places high demands on the operator's abilities.
It is an object of the present invention to provide a steering system which overcomes one or more of the above described problems in the prior art. It is another object to provide a steering system having manual and automatic steering modes which is structurally simplified and at the same time reliable in use.